The present invention relates to a device for automatically adjusting the length of a control cable which operates a mechanical member subjected to dimensional wear, particularly in cases in which the wear affects the operation of a fixed-length control cable.
The above mentioned wear occurs in cables which are used for operating motor vehicle clutches where, because of their extremely frequent use, the anti-slip linings of the clutch discs experience progressive wear. This in turn causes a decrease in the pressure exerted on the movable members, with the result that their capacity to transmit drive to the wheels is seriously impaired. It involves a consequent risk of impairing the performance of the vehicle whose engine loses pulling and braking capacity and very quickly reaches a situation in which considerable repairs are necessary.
It is well known that the clutch control cable has a flexible tubular casing or sheath formed by a tight metal coil which adapts easily to the irregularly shaped compartments of motor vehicle engines, and a thin steel cable which can slide longitudinally in both directions within it. When the control cable is fitted along a curved path and the casing is prevented from moving longitudinally relative to the inner cable, traction or thrust can be transmitted along the path. In order to avoid the harmful effects of wear on the clutch discs, the effective length of the clutch cable should be adjusted continually, either by lengthening of the cable or by shortening of its sheath, so as to compensate for the effects of the wear. The clutches used in the majority of motor vehicles are disc clutches and in general they are associated with a fly wheel which is fixed to the crank shaft of the gear box. They are provided with a spring or springs which keep a pressure plate fitted loosely on the shaft, tight against the outer face of the fly wheel. The driven disc is situated between the fly wheel and the plate and slides on the same gear box shaft. It is gripped between them to transmit the rotation of the engine of the gear box shaft.
In the case in question, the control cable transmits the movement of the clutch pedal to the clutch operating lever which disconnects the engine from the gear box, overcoming the force of the spring or springs which keep the pressure plate, the driven disc and the fly wheel under pressure. The clutch is disengaged to allow gear changing to take place and then reengaged to enable the rotation of the engine to be transmitted to the gear box and the wheels. It is during this clutch reengagement, which is carried out particularly frequently that wear of the anti-slip lining of the disc takes place. As a result, when the wear reaches a certain value, the control cable becomes tensioned so that it has a constant, fixed length and the pressure plate fails to exert the required pressure on the clutch disc. This starts to slip more easily each time and its wear increases progressively, until it fails to fulfill its function of transmitting the rotation of the shaft to the gear box, thereby causing considerable problems and inconvenience, and potentially putting the vehicle and its occupants into dangerous situations. This fault should be rectified before it reaches this extreme condition either by modification of the active length of the control cable if this is possible, or by replacement of the worn driven disc. In both cases, additional expenses and inconveniences are caused to the user.